Electrowetting devices, and in particular electrowetting liquid lenses having a variable focal length, are known in the art, and generally comprise a refractive interface between first and second immiscible liquids that is movable by electrowetting.
FIG. 1 represents FIG. 4 of European Patent Application EP 1662276, and illustrates a variable focus lens 100 according to one example of the prior art. Lens 100 comprises two transparent windows 102, 104, arranged in parallel and facing each other, and delimiting, in part, an internal volume 105 containing two immiscible liquids 106, 108, with different optical indices. Where the two liquids meet they form an optical interface 109 in the form of a meniscus, which can have a number of different shapes, examples being shown by lines A and B. The liquids 106, 108 have substantially equal densities, and one is preferably an insulating liquid, for example comprising oil and/or an oily substance, and the other is preferably a conducting liquid comprising for example an aqueous solution.
Windows 102, 104 may be transparent plates or lenses formed of an optical transparent material such as glass or plastic.
The structure of the lens 100 that contains the liquids comprises a cap 120 to which transparent window 102 is fixed, for example by glue 121, and a body 122 to which transparent window 104 is fixed, for example by glue 123. The cap 120 and body 122 are separated by a gasket 124. Cap 120 comprises a substantially cylindrical side wall 126, while body 122 comprises a substantially cylindrical side wall 127, and gasket 124 is positioned between the side walls 126 and 127, to ensure the tightness of the lens structure.
The cap 120 and body 122 in this example form electrodes of the lens. Cap 120 is used to provide a voltage to the conducting liquid 108. Body 122 is used to provide a voltage close to the edge of the liquid-liquid interface 109. The edge of the liquid-liquid interface 109 contacts an insulated conical surface 126 of the annular body. The parts of body 122 in contact with the liquids 106, 108, including conical surface 126, are coated with an insulating layer (not shown). The interface 109 traversed an opening in the annual body 122 through which light rays can pass.
Due to the electrowetting effect, it is possible, by applying a voltage between the cap 120 and the body 122, to change the positioning of the edge of the liquid-liquid interface on the conical surface 126, and thereby change the curvature of the refractive interface between liquids 106 and 108. For example, the interface may be changed from initial concave shape shown by dashed line A, to a convex shape as shown by solid line B. In this way, rays of light passing through the lens perpendicular to windows 102, 104 in the region of the refractive interface A, B will be focused more or less depending on the voltage applied between the cap 120 and body 122.
In some applications, more complex control of parts of the liquid-liquid interface is needed, for example when image stabilization or when correction of optical aberrations is desirable. Conventionally, image stabilization consists in avoiding blur in images related to vibrations that can occur during the image capture, for example due to hand-shaking, by providing specific devices and/or image processing.
A liquid lens allowing image stabilization has for example been proposed by the applicant in co-pending European application No EP07301180.1. FIG. 2 illustrates such a liquid lens where a refractive liquid-liquid interface 214 can be tilted when two different voltages V1 and V2 are applied between opposite sides of a resistive body 202 and an electrode 216 in contact with a conducting liquid 212. The tilt of the refractive interface 214 allows compensating the movement of the image capture device that includes such a liquid lens.
The present invention aims at providing an improvement of such optical electrowetting devices.